Conventional laser-induced welding techniques of biological tissues have been proposed and studied at an experimental level to assess their possible application in surgery with a view to suturing various types of tissue, e.g., skin, blood vessel walls, nerve tissue and others [K.M. McNally-Heintzelman, “Laser Tissue Welding”, Chap. 39 in: Biomedical Photonics Handbook, pp. 39-1/39-45, T. Vo-Dinh, Ed., CRC Press, Boca Raton (2003)]. These techniques are based generally on the application of laser radiation to the biological tissues, which has the effect of activating certain proteins in the extracellular matrix, such as collagen and elastin, and inducing the immediate adhesion of the wound edges.
The recommended lasers used with this method are generally of the continuous emission type, with wavelengths coming in the visible and infrared spectral bands, e.g. argon, neodymium:YAG, diode and CO2 lasers.
In some cases, the method involves using an exogenous chromophore (such as a biocompatible stain) with a high optical absorption at the wavelength of the laser being used. Said chromophore is applied locally to the edges of the surgical wound to suture and acts as a selective absorber of the laser radiation to enable a more controlled and localized welding effect, minimizing the risk of side-effects such as heat damage to the tissues adjacent to the treatment field.
In ophthalmic surgery, the laser-induced welding of biological tissues can be exploited for various purposes, as an alternative for instance to conventional suturing methods for closing corneal lesions in cataract and corneal transplant surgery [F. Rossi, R. Pini, L. Menabuoni, R. Mencucci, U. Menchini, S. Ambrosini, G. Vannelli, “Experimental study on the healing process following laser welding of the cornea”, Journal of Biomedical Optics 10, pp. 1-7 (2005)].
Various radiation methods are currently used in the laser welding of biological tissues, with and without the aid of a chromophore:
1) direct irradiation, e.g. by means of an articulated arm, especially for laser wavelengths that cannot be transmitted by optical fibers (e.g. those produced by the CO2 laser);
2) operating microscope-guided irradiation, in which case the wound to be welded can be scanned with a beam-splitter used to couple the laser beam to the optical microscope [J. Tang, G. Godlewski, S. Rouy, G. Delacretaz, “Morphologic changes in collagen fibers after 830 nm diode laser welding”, Lasers in Surgery and Medicine 21, pp. 438-443 (1997)];
3) irradiation via optical fibers, keeping the end of the optical fiber emitting the beam a suitable distance from the surface of the tissue (typically a few millimeters away) to avoid soiling the fiber, especially if a stain is used, since this would lead to a substantial reduction in the power transmitted.
One of the problems that remain to be solved in ophthalmgic surgery is how to repair the capsule containing the lens, because the wall of the capsule is extremely slender (10 micrometers) and under considerable tension, so conventional sutures cannot be used. For instance in cataract surgery involving the implantation of an intraocular lens (IOL), the rear wall of the capsule must be preserved in order to prevent the vitreous humor from penetrating into the anterior chamber. One of the most common complications of this type of surgery is represented by the perforation or laceration of the capsule wall as a result of an erroneous manipulation by the surgeon.
The problem of capsule repair is also still without solution in cases of perforating trauma involving the lens, which often gives rise to a severe inflammatory reaction (anaphylactic uveitis), followed by the onset of a post-traumatic cataract. These complications might be avoided if an efficient capsule suturing method were available.
A “Phaco-Ersatz” or “lens refilling” method has recently been proposed for use in ophthalmic surgery, whose purpose is the aspiration of the lens and subsequent refilling of the capsule with a biocompatible polymer that simulates the optical and mechanical properties of the young lens tissue, thereby restoring the transparency and accommodation function of the healthy lens [E. Haefliger, J-M. Parel, “Accommodation of an endocapsular silicone lens (Phaco-Ersatz) in the aging rhesus monkey”, Journal of Refractive Corneal Surgery 10, pp. 550-555 (1994)]. It would have a lot of applications in the treatment of some of the most common ocular disorders, such as presbyopia and lens opacification (cataract). One of the problems still to be solved before the method can be used in clinical practice, however, is how to close the opening in the capsule (or rhexis) used to aspirate the old lens and subsequently refill the capsule.
Mechanical sealing devices have been proposed to solve this problem, such as plastic valves, but these have to be removed at a later date [O. Nishi, K. Nishi, “Accommodation amplitude after lens refilling with injectable silicone by sealing the capsule with a plug in primates”, Archives of Ophthalmology 116, pp. 1358-61 (1998)]. Such mechanical valves have been tested at experimental level in an animal model to fill the capsule without the polymer leaking into the anterior chamber, but they would hardly be suitable for use in humans because, if they were left in place, they would become a foreign body that would partially obstruct the vision, interfering with lens accommodation and giving rise to various inflammatory or rejection processes during the healing period. Moreover, removing them—after the polymer has been irradiated from the outside to ensure its polymerization—would involve accessing the anterior chamber again and, in any case, the rhexis in the capsule would be left open.